The production of filaments and filament yams generally takes place in accordance with the melt-spinning method.
Based on a molten flow, which is delivered from an extruder or directly from the poly-condensation installation, the polymer is distributed to the individual spinning spinnerets by spinning pumps. After the melt exits the capillary bores of the spinnerets in the form of fine filaments, these are cooled by means of a cooling medium, thereafter gathered or bundled, treated with spinning preparations and wound.
At the beginning of the development of melt spinning methods, the spun filaments were cooled without the active support of a device only by means of their own vertical movement in the air medium on their way to being wound.
Since the mid-fifties, active cooling systems have been employed, primarily with the use of cross-flow air diffusion, to reduce the height of the machines and to increase the capacity.
Cooling of the filaments is a very essential step in the total process of producing a polymer filament. Uniformity of mass, the quality of dye absorption, as well as the textile properties, such as strength and stretching, are affected by this.
The past ten years have seen a development in spinning technology for producing filament yarns with still finer titers of individual filaments, so-called microfilaments, of a linear density below 1 dtex per filament.
The filament yarns which are customarily used for further textile production, having a total titer of 84 dtex, or respectively 167 dtex, are then no longer composed of 36, or respectively 72 filaments, but in accordance with the present state of the art of approximately 100 to 200 individual filaments.
Products made of so many microfilaments are distinguished by special properties, which are to the advantage of the consumer.
For cooling filaments or threads after melt spinning, customarily a so-called cross-flow air diffuser method is used in accordance with the state of the art. However, this makes it necessary to employ spinnerets of large diameter for yarns with high filament counts, since, for reasons of uniformity of the product, with filament cooling by means of these methods it is not possible to exceed hole densities of approximately 8 holes/cm.sup.2 on the spinneret.
However, large spinnerets result in disadvantages in regard to space requirements of the production installations and in regard to product quality because of the increasingly non-uniform temperature over the surface of the spinneret, as well as the increased dwell time of the polymer melt in the nozzle package.
Devices which have been shown to be particularly suited to spinning multi-capillary products are known, for example, from DE 36 29 731 A1, DE 196 53 451 C1 or WO 92/15732 A1.
In these devices the filaments are cooled by means of a central air diffuser system after exiting the spinneret. To this end the filaments are spun from a spinneret, whose capillary bores or holes are arranged in one or several, preferably concentric, circles. The diameter of the smallest circle must be sufficiently large in order to be able to install the cooling device, the so-called air flow candle, centered underneath the spinning device. To this air flow candle, consisting of a tube-shaped, porous gas-permeable hollow body, air is supplied from one tube end, the oppositely located tube end is closed. The cooling air flows radially outward through the porous candle and in this way cools the filaments, which are arranged concentrically around it. After passage through the air diffusion zone, the filaments graze a ring for the application of the spinning preparation. Thereafter they are combined into a strand underneath the air flow candle. The filaments spun in this way are suitable for producing staple fibers.
A central air diffusion process for producing technical yarns, which are distinguished by low shrinkage and a high modulus, from a large number of individual filaments with large capillary titers of more than 1 dtex/fil is claimed in DE 196 53 451 C1.
The production of technical polyester yarn with a low titer while using a central cooling unit, which starts with a zone of a length of approximately 15 to 60 cm, through which no air is diffused, but which is heated from the outside in order to improve the Uster uniformity of the yarn, is described in U.S. Pat. No. 3,969,462.
A central air diffusing device with an annular die slot screen, which is arranged between the spinneret and the air flow candle for preventing interference with spinning, is described in DE 38 22 571 A1. It is pointed out that in actual use, without such an arrangement there are frequent interruptions of the operation because of filament breaks, and the mass uniformity of the filaments remains unsatisfactory in comparison with cross-flow air diffusion.
Although the devices known from the prior art have proven themselves for producing products at a high rate of throughput, such as are necessary for the production of staple fibers and of yarns for technical applications, they are insufficient for the production of microfilament endless yarns, wherein the throughput per nozzle is considerably less. Clear disadvantages occur in connection with the production of microfilament yarns, which will be described in greater detail below:
The spinning of microfilaments for textile yarns is in no way a trivial undertaking for one skilled in the art. As known from the prior art, in connection with such products the danger arises that the spinneret cools because of the low melt throughput and spinning problems occur increasingly because of this, such as described by Th. Tekaat in "Chemiefasern/Textilindustrie" [Chemical Fibers/Textile Industry], 42/194, p. 879.
Therefore the known devices were only employed for large titers clearly above 1 dtex/fil, or in fiber spinning methods with a very high hole count per spinneret. In DE 37 08 168 C2, for example, more than 700 holes per spinneret are mentioned. In fiber spinning methods the spinneret is provided with sufficient heat by the molten mass because of the required high melt throughput.
In order to overcome the cooling of the spinneret during the production of microfilaments, one skilled in the art then makes use of higher spinning, or respectively melt temperatures, in particular in the case of cross-flow air diffusion. However, higher temperatures adversely affect the dependability of the process to a considerable degree, or respectively the frequency of interruptions is increased because of the increased thermal decomposition of the polymer melt in the melt supply system, the so called "spin beam" and nozzle package, and of the increasing contamination on the surface of the spinneret.
A device for the passive cooling of spun filaments is described in the unpublished DE patent document DE 197 16 394.7-26, by means of which it is only possible to achieve hole counts of maximally 300 for spinnerets of customary size with diameters of up to 110 mm and only hole densities around 10 holes/cm.sup.2
A hole density of only maximally 25 holes/cm5 is achieved for spinnerets with a circular-shaped arrangement of holes in EP 0 646 198 B1.
Accordingly, the upper limit of the hole density known from the prior art remains below 30 holes/cm.sup.2. Higher hole densities can also not be achieved with this device without losses in quality and an increase of spinning problems.
The devices and methods described in the prior art cannot achieve this goal.